Transparent resin films have been attempted to be used in recent years as an alternative to glass substrates in applications such as image display elements or solar cell modules accompanying requirements for increased flexibility, lighter weight and reduced thickness.
FIG. 5 is a schematic cross-sectional view showing an example of a solar cell module in which a transparent resin film is used as a protective sheet.
This solar cell module 200 is generally composed of a solar cell 201 composed of crystalline silicon or amorphous silicon and the like, an encapsulant (filler layer) composed of an electrical insulator that seals the solar cell 201, a front protective sheet (front sheet) 203 laminated onto the front of the encapsulant 202, and a back protective sheet (back sheet) 204 laminated onto the back of the encapsulant 202.
In order to impart weather resistance and durability to the solar cell module 200 enabling it to withstand outdoor and indoor use over a long period of time, it is necessary to protect the solar cell module 201 and the encapsulant 202 from wind and rain, humidity, debris and mechanical impacts while also maintaining the inside of the solar cell module 200 in a sealed state that is completely isolated from the outside air. Consequently, the front sheet 203 and the back sheet 204 are required to have superior weather resistance, and are required to have a low level of water vapor permeability (superior gas barrier properties) in particular.
However, transparent resin films have poor gas barrier properties in comparison with glass substrates, and are unable to adequately prevent entrance of water vapor into solar modules as is. In addition, since transparent resin films have low surface flatness, when an electrode film is deposited on the surface thereof, protrusions that cause disconnections or short circuits are formed in the electrode film.
Therefore, a film (gas barrier film) has been proposed that is imparted with gas barrier properties by laminating an inorganic thin film having gas barrier properties onto a synthetic resin film by sputtering method and the like (see, for example, Patent Document 1).
However, although conventional gas barrier films have improved gas barrier properties in comparison with transparent resin films alone, they are still considered to be adequately satisfactory. In addition, the inorganic thin film is subject to the occurrence of cracking when the gas barrier films are bent, thereby resulting in the risk of gas easily permeating the film at locations where cracking has occurred.
In order to solve this problem of the occurrence of cracking caused by bending of the film, a technique has been proposed that makes the inorganic thin film resistant to cracking by alternately laminating an inorganic thin film and organic thin film on a synthetic resin sheet (see, for example, Patent Document 2). However, in addition the production process of this film being complex, adhesion between the inorganic thin film and organic thin film is low, thereby resulting in increased susceptibility to separation of the thin films.